V INTERACTION OF FACTORS 43 



influence one another in the zygote. The rose and the 

 pea factors are separate entities, and each when present 

 alone produces a perfectly distinct and characteristic 

 effect upon the single comb, turning it into a rose or a pea 

 as the case may be. But when both are present in the 

 same zygote their combined effect is to produce the wal- 

 nut comb, a comb which is quite distinct from either and 

 in no sense intermediate between them. The question 

 of the influence of factors upon one another did not pre- 

 sent itself to Mendel because he worked with characters 

 which affected different parts of the plant. It was un- 

 likely that the factor which led to the production of 

 colour in the flower would affect the shape of the pod, or 

 that the height of the plant would be influenced by 

 presence or absence of the factor that determined the 

 shape of the ripe seed. But when several factors can 

 modify the same structure it is reasonable to suppose that 

 they will influence one another in the effects which their 

 simultaneous presence has upon the zygote. By them- 

 selves the pea and the rose factors each produce a definite 

 modification of the single comb, but when both are pres- 

 ent in the zygote, whether as a single or double dose, the 

 modification that results is quite different to that pro- 

 duced by either when present alone. Thus we are led 

 to the conception of characters which depend for their 

 manifestation on more than one factor in the zygote, and 

 in the present chapter we may consider a few of the 



